1. Field of the Invention
The present invention relates generally to programmable logic circuitry, and more particularly, to the use of EPROM or EEPROM transistor elements in programmable integrated circuit logic array devices for the purpose of architecture control.
2. Description of the Prior Art
In the use of integrated circuit devices (ICs) it is desirable that the devices be programmable so that their operation may be tailored to fit particular applications. Furthermore, it is desirable that a menu of features be provided from which an end user of the IC can choose to best suit his particular application. Selection of items from a menu can be described as configuring the architecture of the IC and various means of selecting IC architectures are currently used. Two commonly used methods are (1) to provide mask options and (2) to provide "mode" or "control" registers (storage elements) whose register outputs act as architecture controls. However, each of these methods have associated disadvantages.
The first method referred to, i.e. wherein masks used to manufacture the IC are programmed, has the drawback that the configuration of the IC must be implemented during the manufacturing process. Typically, such programming is accomplished during the metal masking step in the production of the IC. The disadvantage of this approach is that configuration selections must be made prior to completion of the manufacturing operation, thereby causing long lead times and many production and inventory problems associated with manufacturing, testing and inventorying the custom IC devices. In addition if changes are later required all previous inventory that has matured beyond the point in manufacturing where customization takes place will be obsolete and thus useless.
The second method, i.e. the use of mode registers, solved the lead time, manufacturing and inventory problems in that the IC parts could be configured by the user after manufacture was completed. In other words, if changes were required, the IC device could be reprogrammed by writing new values into the mode register. The disadvantage of this approach is that register data is lost when power is removed from the system. As a consequence, applications of devices having such feature are limited.